Recently, a light emitting diode (LED) that emits the light with a short wavelength, including a blue LED or UV LED, has been developed and commercialized. The use of such an LED has been extended rapidly, and particular examples of the use include general lighting in which the conventional fluorescent lamps or light bulbs are used and light sources in which the conventional short arc lamps are used for curing UV curable resins or UV curable ink.
In general, an LED has an LED die including an anode and cathode formed on the surface thereof. Each of the anode and cathode is wire bonded to the external electrode and the LED die electrically communicates with the external electrode to carry out light emission.
In the LED having the above-mentioned constitution, when the LED die and fine wires (e.g. diameter: 30 μm) are exposed to the external space, the LED die may be damaged or the wires may undergo a short. Thus, the LED is used generally after it is sealed with a sealant (such as a resin).
In addition, When the LED is sealed with a sealant having a refraction index higher than the refraction index of air, the difference in refraction index at the interface between the LED die and sealant is decreased. Therefore, sealing the LED with a sealant is also effective for improving the light ejection efficiency.
In the case of the conventional LED emitting the visible light, a highly transparent epoxy resin, silicone resin, etc. have been used as a sealant (see, Patent Documents 1 and 2). However, when using the conventional epoxy resin or silicone resin is applied to an LED emitting the light with a short wavelength, the resin itself may be degraded due to the short-wavelength light, thereby causing some problems, such as coloration of cracking. In addition, such problems of a sealant become serious particularly in the case of a UV LED used as a light source emitting strong UV rays for curing a UV curable resin or UV curable ink.
For example, the UV LED used as a light source for curing a UV curable resin or UV curable ink includes an LED emitting 1 W of UV light having a wavelength of 365 nm through the supply of an electric power of 3 W to an LED die having a width×length of 1 mm×1 mm. In this case, the light irradiation dose becomes 1 W/mm2, which corresponds to 30,000-50,000 times of the UV light dose included in the solar light. Due to this, it is required for a sealant for an UV LED as a light source for curing a UV curable resin or UV curable ink to have resistance against strong UV rays in addition to high transparency in a wavelength region of light emission for the UV LED.
In addition, since 2 W of the electric power of 3 W inputted to the UV LED is converted into thermal energy so that the LED die itself may be heated, it is required for the sealant for an UV LED as a light source for curing a UV curable resin or UV curable ink to have resistance against heat (temperature) in addition to UV ray resistance.
Further, it has been suggested that a composition including an epoxy group-containing multifunctional polysiloxane and a metal chelate compound is used as a sealant for an LED or the like (Patent Document 3).